How to round a number in JavaScript only if necessary

Question

I want to round a number in JavaScript but to 2 decimal places but only if necessary.

For example 4.5 to be 4.50 and 3.331 to be 3.331.

I have tried using Math.round(number / 100) * 100 but when number is 4.5 it gives me 4.5 and I want it to be 4.50.

I have tried using .toFixed(2), but when number is 3.331 it will fix it to 3.33.

Any help is much appreciated.

Possible duplicate of [Round to at most 2 decimal places (only if necessary)](https://stackoverflow.com/questions/11832914/round-to-at-most-2-decimal-places-only-if-necessary) — Devsi Odedra, Sep 12 '19 at 11:15
That's not rounding a number - that's padding it. Rounding means that you remove some of the least digits and adjust the remainder up or down to account for them. — VLAZ, Sep 12 '19 at 11:17
It doesn't look like a rounding problem. In the end, you're just padding with zeroes when the number has less than two decimals. — Robby Cornelissen, Sep 12 '19 at 11:17
@NinaScholz I think the logic is - have *at least* two decimal places but if you have more, keep them. So, it's not actually rounding. — VLAZ, Sep 12 '19 at 11:20

score 3 · Accepted Answer · answered Sep 12 '19 at 11:23

3

You could check if the fixed value change and select the kind of formatting.

function format(f) {
    return f === +f.toFixed(2)
        ? f.toFixed(2)
        : f.toString()
}

console.log([4.5, 3.331, 3].map(format));

answered Sep 12 '19 at 11:23

Nina Scholz

376,160
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Janie · Answer 2 · 2019-09-12T11:33:02.790

1

Using regex check the digits after the dot. if it is less than 2 then use toFixed(2), else do nothing.

function f(n) {
    let diff = Number(n) - Math.floor(Number(n));
    return /\.[0-9]{2,}/.test(diff.toString()) ? n : n.toFixed(2)
}
const nums = [3.331, 2.5, 3].map(f)
console.log(nums)

edited Sep 12 '19 at 11:33

answered Sep 12 '19 at 11:21

Janie

638
9
26

What about `num = 3`? – VLAZ Sep 12 '19 at 11:21
Since OP didn't ask it, that's why I ignored it. – Janie Sep 12 '19 at 11:24
The new solution doesn't work like before - `2.5` turns into `2.5` – VLAZ Sep 12 '19 at 11:25
Now it returns different types - sometimes it's an integer, other times a string... – VLAZ Sep 12 '19 at 11:38

score 0 · Answer 3 · answered Sep 12 '19 at 11:28

You can try to apply conditionallu toFixed() method:

const numbers = [4.5, 3.331];
const floated = numbers.map(
  n => {
    const floatLen = n.toString().split('.')[1];
    if(floatLen){
      if(floatLen.toString().length < 2){
        return n.toFixed(2);
      }
    }
  return n;
  }
);
console.log(floated);

Phyrum Tea · Answer 4 · 2019-09-12T12:26:18.693

0

This looks like an String format and not a rounding problem:

var value = 4 + '.';
var [num, dec] = value.split('.');

if (dec.length < 2) {
  dec = (dec + '00').substring(0, 2);
}
console.log(`${num}.${dec}`);

Runnable snippet:

function format(value) {
  var [num, decimal] = (value + '.').split('.');
  
  if (decimal.length < 2) {
    decimal = (decimal + '00').substring(0, 2);
  }

  return `${num}.${decimal}`;  
}

console.log([4, 4.5, 3.331].map(format));

4 -> 4.00

4.5 -> 4.50

3.331 -> 3.331

edited Sep 12 '19 at 12:26

answered Sep 12 '19 at 11:44

Phyrum Tea

2,623
1
19
20

Not directly related but `num` and `digit` are confusing names. A digit *is* a number. Also, having a digit with any length that's not `1` is impossible. – VLAZ Sep 12 '19 at 11:50
I don't see the limit to 1, have to rename the variable names – Phyrum Tea Sep 12 '19 at 11:56
A *digit* is a single symbol that creates a number - `1`, `2`, `3`, `4`, `5`, `6`, `7`, `8`, `9`, and `0` are all the digits in decimal. Hex also has `a`, `b`, `c`, `d`, `e`, `f`, for example. A number is composed of one or more digits, so each of these. That's why you have a "two digit number" like `42` or `55`, or "three digit number" - `123`, `216`, `715`, etc. – VLAZ Sep 12 '19 at 12:00
I see, you not referring to the variable. – Phyrum Tea Sep 12 '19 at 12:06

How to round a number in JavaScript only if necessary

4 Answers4